Method for end-capping polycarbonate resins and composition for use in same

ABSTRACT

A method for end-capping polycarbonate resins, comprising the step of processing a mixture comprising a polycarbonate having free hydroxyl-end groups and an end-capping reagent in a melt transesterification reaction to produce a polycarbonate resin, wherein the end-capping reagent comprises a mixture of:(a) at least one species of a symmetrical activated aromatic carbonate, and (b) at least one species of a symmetrical non-activated aromatic carbonate, whereby said end-capping reagent reacts with at least some of the free hydroxyl end-groups of the polycarbonate to produce an end-capped polycarbonate resin.

BACKGROUND OF INVENTION

This application relates to a method for end-capping polycarbonateresins and to end-capping compositions useful in such a method.

Polycarbonates prepared by the reaction of a dihydric phenol (such asbisphenol A, “BPA”) and a diaryl carbonate (such as diphenyl carbonate,“DPC”) in a melt transesterification process generally containsignificant levels of uncapped chains (7-50%) as compared tointerfacially prepared polycarbonates. These uncapped chains can have asignificant impact on the resulting properties of polycarbonate, and itis therefore desirable in many instances to include an end-capping agentwith a higher capping efficiency than DPC during or after thepolymerization reaction which terminates the uncapped chains.

Known end-capping reagents are frequently carbonate or ester derivativesof phenol. U.S. Pat. No. 4,680,372 discloses the use of phenyl benzoateas an end-capping reagent to terminate polymers formed by meltpolymerization of a bisphenol and an aromatic dicarboxylic acid such asterephthalic acid and/or isophthalic acid. U.S. Pat. No. 4,886,875describes preparation of polyarylate compositions using diarylcarbonates, polyarylcarbonate oligomers or polyarylcarbonate polymers asend-capping agents. In particular, the Examples of the ″875 patent teachthe use of diphenyl carbonate or highly endcapped polycarbonateoligomers as end-capping agents. Unfortunately these end-cappingreagents all yield the byproduct phenol, which then rapidlyre-equilibrates with the polycarbonate to limit the achievable molarmass and end-cap level. Long reaction and devolatization times arerequired to counteract this effect.

Therefore known end-capping reagents are frequently also carbonate orester derivatives of electronegatively-ortho-substituted phenols whichare more reactive than DPC. U.S. Pat. No. 4,310,656 describes thetransesterification of of bis (ortho-haloaryl)carbonates, haloaryl arylcarbonates, and a dihydric phenol, and indicates that controlled arylend-capping is achieved. U.S. Pat. No. 4,363,905 describes thetransesterification of of bis(ortho-nitroaryl)carbonates, nitro arylaryl carbonates and a dihydric phenol, and indicates that controlledaryl end-capping is achieved. It should be noted though that bothbis(ortho-haloaryl)carbonates and bis(ortho-nitroaryl)carbonates havequite different properties than DPC. Thus the replacement of DPC bythese compounds requires considerably different equipment and operatingconditions than typically found in melt polycarbonate production. Inaddition The use of these compounds results in the production of coloredor potentially toxic or explosive byproducts or ones that producegaseous products containing chlorine upon combustion. Thus from productquality (transparency), handling, and environmental considerations thereis a demand for the use of carbonates that are free from chlorine andnitro-activating groups. U.S. Pat. No. 4,661,567 describes the use ofvinylene carbonate derivatives as end-capping agents which are added topreformed polycarbonates to terminate the polymers.

U.S. Pat. No. 5,696,222 describes a process for production of aterminally-blocked polycarbonate by melt transesterification of adihydric phenol and a diaryl carbonate in the presence of an asymmetricsubstituted phenol ester or carbonate as an end-capping agent, and inparticular end-capping agents which are salicylic acid derivatives.European Patent Publication No. 0 980 861 discloses an improved methodfor making such derivatives. These end-capping agents are derived fromone salicylate (activated) and one non-activated phenol. While suchend-capping agents are effective, they are not without their drawbacks.Specifically, such asymmetric carbonates require two separate steps fortheir preparation (generation of a chloroformate from one of thephenols, followed by condensation with the second phenol). This two stepprocess adds significantly to the cost of the end-capping agent. Anadditional deficiency of this method is that the asymmetric mixedcarbonates prepared in this way are often contaminated with traces ofnitrogen- and halogen-containing impurities and with symmetricalcarbonates derived from one or both of the phenols used in the reaction.As a consequence, in order to obtain materials of suitable quality forpolymerization, purification is often both essential and difficult.

The ″222 patent also describes the use of di-activated carbonates, forexample derived from two salicylates to either couple polycarbonatechains to increase molecular weight, or to cap phenolic hydroxyl endgroups. This method suffers from the fact that only salicylate(activated) end groups can be incorporated. These salicylate end groupsare different from the conventional phenol or alkyl-substituted phenolend groups typically found in commercial polycarbonate resins. Inaddition, capping is accompanied by coupling so that it is difficult toonly cap without increasing molecular weight, or to systematically varythe endcap level without varying also the polycarbonate molecularweight. Indeed, while increasing levels of conventional end-cappingagents tend to decrease molecular weight, in the case of thedi-activated species there are opposing tendencies where the end-cappingfunction reduces molecular weight while the coupling function tends toincrease molecular weight, making the characteristics of the productdifficult to predict or control.

SUMMARY OF INVENTION

It has now been found that end-capping reagents which comprise a mixtureof different species, including at least: (a) a symmetrical activatedaromatic carbonate, and (b) a symmetrical non-activated aromaticcarbonateprovide effective end-capping of polycarbonate resins and canavoid the deficiencies described above. Thus, the present inventionprovides an end-capping reagent, and a method for preparing anend-capped polycarbonate resin using the reagent. In accordance with anembodiment of the method of the invention a mixture comprising apolycarbonate having free hydroxyl-end groups and an end-capping reagentis processed in a melt transesterification reaction to produce apolycarbonate resin.

The carbonates of the end-capping reagent reacts with at least some ofthe free hydroxyl end-groups of the polycarbonate to produce anend-capped polycarbonate resin.

DETAILED DESCRIPTION

End-Capping Reagent: This application relates to an end-cappingcomposition comprising a mixture of different species, including atleast: (a) one species of symmetrical activated aromatic carbonate, and(b) one species of symmetrical non-activated aromatic carbonate.

This invention further relates to a method of making polycarbonate resinwhich uses an end-capping reagent of this type.

As used in the specification and claims of this application, the term“symmetrical activated aromatic carbonate” refers to compoundscontaining two phenolic groups linked through a carbonate bridge, witheach phenol group being substituted with the same electronegative andtherefore activating substituent at the ortho position. Many of thesesymmetrical activated aromatic carbonates can be represented by thegeneral formula:

wherein R is the electronegative substituent. Preferred electronegativesubstituents are carbonyl-containing groups, nitro groups, and halogroups. Symmetrical activated aromatic carbonates of this type may besynthesized by the reaction of an appropriate ortho-substituted phenolwith phosgene.

Symmetrical activated aromatic carbonates for use in compositions andmethods in accordance with the invention may also be made either byreaction of two equivalents of an appropriate “activated” orortho-substituted phenyl chloroformate with a bisphenol, such asbisphenol A, or by reaction of a bis-chloroformate with two equivalentsof an “activated” or appropriate ortho-substituted phenol. Wherebisphenol A is used, the resulting composition has the following generalformula,

wherein R is as defined above.

Specific examples of symmetrical activated aromatic carbonates which maybe used in the compositions and methods of the invention are summarizedin Table 1.

As used in the specification and claims of this application, the term“symmetrical non-activated aromatic carbonate” refers to compoundscontaining two phenolic groups linked through a carbonate bridge. Thephenol groups may be substituted or unsubstituted, provided that they donot include electronegative and therefore activating substituents at theortho position. Suitable symmetrical and asymmetrical non-activatedaromatic carbonates may be represented a compound of the formula:

wherein the R₂ groups are selected from the group consisting of hydrogenand a C₁-C₃₆ alkyl, C₁-C₃₆ alkoxy group, C₆-C₃₆. aryl, C₇-C₃₆ aralkyl,and C₇-C₃₆ aralkyloxy and n is selected from the integers 1-5. Specificsymmetrical non-limiting examples of nonactivated aromatic carbonateswhich may be used in the compositions and methods of the inventioninclude diphenylcarbonate, di-p-cumylphenylcarbonate, di-(tert-butylphenyl) carbonate, di-(octylphenyl) carbonate, di-(nonylphenyl)carbonate, di-(dodecylphenyl) carbonate, di-(3-pentadecyl)phenylcarbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.

Although it will generally increase the cost without providing acompensating benefit, in addition to the symmetrical carbonates, theend-capping reagent may also include asymmetric activated ornon-activated carbonates. Examples of asymmetrical non-activatedcarbonates include p-cumylphenyl phenyl carbonate, tert-butylphenylphenyl carbonate, octylphenyl phenyl carbonate, nonylphenyl phenylcarbonate, dodecylphenyl phenyl carbonate, 3-pentadecylphenyl phenylcarbonate. Exemplary asymmetrical activated carbonates have the generalformula

wherein R is an electronegative substituent and R₂ is selected fromamong hydrogen, C₁ to C₁₈ alkyl, C₁ to C₁₈ alkoxy, C₆ to C₁₈ aryl, C₇ toC₁₈ aralkyl, and C₇ to C₁₈ aralkoxy. The mixture of the symmetricalactivated aromatic carbonate andsymmetrical non-activated aromaticcarbonate may be preformed prior to addition to the melttransesterification reaction, or may be generated in situ by separateaddition of the carbonate species to the melt transesterification. Thismixture, whether preformed or generated in situ, is referred to hereinas the “end-capping reagent.” In the case of a pre-formed end-cappingreagent, the mixture may be neat, or it may include a suitable solvent,for example PETS, or a mixture of toluene and acetone.

The relative amounts of the two types of carbonates (activated andnon-activated) in the end-capping reagent can be varied depending on theproduct characteristics desired. Higher amounts of activated carbonatewill increase the degree of coupling obtained, while higher relativeamounts of the non-activated carbonate can be used to enhance the amountof chain termination. In general, the mole ratio of non-activated toactivated carbonates will suitably range from 10:90 to 90:10.

Preparation of the end-capping reagent In one embodiment of theinvention, the symmetrical carbonates (both activated and non-activated)are prepared using a method in which two equivalents of a substitutedphenol is reacted with one equivalent of phosgene in an interfacialreaction using water and a chlorinated solvent such as methylenechloride and in the presence of a base such as sodium hydroxide toneutralize the liberated HCl. Additional catalysts may be employed inthis reaction to facilitate the condensation reaction. In oneembodiment, the condensation catalyst is triethyl amine, quaternaryalkyl ammonium salt, or mixtures thereof. After completion of thecondensation reaction, the organic product phase is washed with aqueousacid and then with water until the washings are neutral. The organicsolvent may be removed by distillation and the end-capper iscrystallized or distilled and recovered.

End-capping Reaction in the Polycarbonate Production Process: Theend-capping reagent of the present invention is used to rapidly cap theterminal hydroxy group of the polycarbonate. The ortho-substitutedphenols generated in the capping reaction are less reactive than phenolin backbiting reactions, which lead to molecular weight degradation ofthe polycarbonate. Therefore, the by-product phenols are removed fromthe terminal-blocked polycarbonate by distillation to the over-headsystem using conventional means (i.e., freeze traps using chilled wateras a coolant) where they can be condensed to expedite the end-cappingreaction at high yields.

It should be noted that the end-capped polycarbonate may still containsmall amounts of any unrecovered phenols, any unreacted end-cappingreagent along with by-products of any side reactions to the end-cappingreactions, e.g. terminal 2-(alkoxycarbonyl)phenyl groups and the like.In one embodiment, the end-capped polycarbonate contains about less than500 ppm of ortho-substituted phenols and about 500 ppm of unreactedterminal blocking agent of the present invention. In another embodiment,the end-capped polycarbonate contains about 2,500 ppm or less ofterminal 2-(alkoxycarbonyl)phenyl groups.

In one embodiment, the ortho-substituted phenol by-product of thefollowing formula is recovered from the overhead system and reused toprepare new end-capping reagents.

In accordance with the method of the invention, end-capping reagentcontaining activated and non-activated carbonates is combined with apreformed polycarbonate polymer having free hydroxyl end groups. Thepreformed polycarbonate polymer may be any type of polycarbonate, andcan be formed by either melt transesterification or an interfacialprocess, although most commonly the preformed polycarbonate polymerwould be formed from the melt transesterification process.

Melt Polycarbonate Process The process of the present invention is amelt or transesterification process. The production of polycarbonates bytransesterification is well-known in the art and described, for example,in Organic Polymer Chemistry by K. J. Saunders, 1973, Chapman and HallLtd., as well as in a number of U.S. patents, including U.S. Pat. Nos.3,442,854; 5,026,817; 5,097,002; 5,142,018; 5,151,491; and 5,340,905.

In the melt process, polycarbonate is produced by the meltpolycondensation of aromatic dihydroxy compounds (A) and carbonic aciddiesters (B). The reaction can be carried out by either a batch mode ora continuous mode. The apparatus in which the reaction is carried outcan be any suitable type of tank, tube, or column. The continuousprocesses usually involve the use of one or more CSTR's and one or morefinishing reactors.

Examples of the aromatic dihydroxy compounds (A) includebis(hydroxyaryl) alkanes such as bis(4-hydroxyphenyl)methane; 1,1-bis(4-hydroxyphenyl)ethane; 2,2-bis(4-hydroxyphenyl)propane (also knownas bisphenol A); 2,2-bis(4-hydroxyphenyl)butane;2,2-bis(4-hydroxyphenyl)octane; bis(4-hydroxyphenyl)phenylmethane;2,2-bis(4-hydroxy-1-methylphenyl)propane;1,1-bis(4-hydroxy-t-butylphenyl)propane; and2,2-bis(4-hydroxy-3-bromophenyl)propane; bis (hydroxyaryl)cycloalkanessuch as 1,1-(4-hydroxyphenyl)cyclopentane and1,1-bis(4-hydroxyphenyl)cyclohexane; dihydroxyaryl ethers such as4,4′-dihydroxydiphenyl ether and 4,4′dihydroxy-3,3′-dimethylphenylether; dihydroxydiaryl sulfides such as 4,4′-dihydroxydiphenyl sulfideand 4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfide; dihydroxydiarylsulfoxides such as 4,4′-dihydroxydiphenyl sulfoxide and4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfoxide; and dihydroxydiarylsulfones such as 4,4′-dihydroxydiphenyl sulfone and4,4′-dihydroxy-3,3′-dimethyldiphenyl sulfone. In one embodiment, thearomatic dihydroxy compound is bisphenol A (BPA).

Examples of the carbonic acid diesters (B) include diphenyl carbonate;ditolyl carbonate; bis(chlorophenyl)carbonate; m-cresyl carbonnate; anddinaphthyl carbonate. In one embodiment of an industrial process,diphenyl carbonate (DPC) is used.

The carbonic diester component may also contain a minor amount, e.g., upto about 50 mole % of a dicarboxylic acid or its ester, such asterephthalic acid or diphenyl isophthalate, to preparepolyesterpolycarbonates. In preparing the polycarbonates, usually about1.0 mole to about 1.30 moles of carbonic diester are utilized for every1 mole of the aromatic dihydroxy compound. In one embodiment, about 1.01moles to about 1.20 moles of the carbonic diester is utilized.

Optional Terminators/End-capping Agents. In one embodiment of the meltprocess, additional/optional terminators or end-capping agents of theprior art may also be used. Examples of terminators include phenol,p-tert-butylphenol, p-cumylphenol, octylphenol, nonylphenol and otherend-capping agents well-known in the art.

Optional Branching Agents. In one embodiment of the process of thepresent invention, branching agents are used as needed. Branching agentsare well-known and may comprise polyfunctional organic compoundscontaining at least three functional groups, which may be hydroxyl,carboxyl, carboxylic anhydride, and mixtures thereof. Specific examplesinclude trimellitic anhydride, tris-p-hydroxy phenyl ethane,isatin-bis-phenol, tris-phenol TC (1,3,5-tris((p-hydroxyphenyl)isopropyl)benzene), tris-phenol PA (4(4(1,1-bis(p-hydroxyphenyl)-ethyl)alpha,alpha-dimethyl benzyl)phenol, and benzophenone tetracarboxylicacid. Optional catalysts. The polycarbonate synthesis may be conductedin the presence of a catalyst to promote the transesterificationreaction. Examples include alkali metals and alkaline earth metals bythemselves or as oxides, hydroxides, amide compounds, alcoholates, andphenolates, basic metal oxides such as ZnO, PbO, and Sb₂ O₃,organotitanium compounds, soluble manganese compounds,nitrogen-containing basic compounds and acetates of calcium, magnesium,zinc, lead, tin, manganese, cadmium, and cobalt, and compound catalystsystems such as a nitrogen-containing basic compound and a boroncompound, a nitrogen-containing basic compound and an alkali (alkalineearth) metal compound, and a nitrogen-containing basic compound, analkali (alkaline earth) metal compound, and a boron compound.

In one embodiment of the invention, the transesterification catalyst isa quaternary ammonium compound or a quaternary phosphonium compound.Non-limiting examples of these compounds include tetramethyl ammoniumhydroxide, tetramethyl ammonium acetate, tetramethyl ammonium fluoride,tetramethyl ammonium tetraphenyl borate, tetraphenyl phosphoniumfluoride, tetraphenyl phosphonium tetraphenyl borate, tetrabutylphosphonium hydroxide, tetrabutyl phosphonium acetate and dimethyldiphenyl ammonium hydroxide.

The above-mentioned catalysts may each be used by themselves, or,depending on the intended use, two or more types may be used incombination. When more than one catalyst is employed, each may beincorporated into the melt at a different stage of the reaction. In oneembodiment of the invention, part or all of one catalyst is addedtogether with the end-capping reagent.

The appropriate level of catalyst will depend in part on how manycatalysts are being employed, e.g., one or two. In general, the totalamount of catalyst is usually in the range of about 1×10⁻⁸ to about 1.0mole per mole of the dihydroxy compound. In one embodiment, the level isin the range of about 1×10⁻⁵ to about 5×10⁻² mole per mole of dihydroxycompound. When more than one catalyst is employed, each may beincorporated into the melt at a different stage of the reaction.

Other Optional Components in the Polycarbonate

In the present invention, the polycarbonate obtained may further containat least one of a heat stabilizer, an ultraviolet absorbent, a moldreleasing agent, a colorant, an anti-static agent, a lubricant, ananti-fogging agent, a natural oil, a synthetic oil, a wax, an organicfiller and an inorganic filler, which are generally used in the art.

Adding the end-capping agent to the melt process The method of addingthe end-capping agent to polycarbonate is not specially limited. Forexample, the end-capping agent may be added to the polycarbonate as areaction product in a batch reactor or a continuous reactor system. Inone embodiment, the end-capping agent is added to the melt polycarbonatejust before or after a later reactor, i.e., a polymerizer, in acontinuous reactor system. In a second embodiment, the end-capping agentis by reactive extrusion after the last polymerizer in the continuousreactor system. In a third embodiment, it is added between the 1^(st)and 2^(nd) polymerizer in a continuous reactor system. In yet anotherembodiment, the end-capping agent is added between the 2^(nd) reactorand the 1^(st) polymerizer.

The amount of end-capping reagent appropriately utilized can bequantified with reference to the amount of free hydroxyl end groups inthe pre-formed polycarbonate polymer. In general, the mole ratio oftotal carbonate to free hydroxyl end groups is within the range of from0.5 to 2.0, depending on the level of end-capping desired.

In some reactor systems, it may be difficult to reach the desiredequilibrium ratio of introduced or incorporated non-activated andactivated end-groups (the non-activated end groups are preferentiallypresent at equilibrium) due to poor mixing, short residence time, orrapid volatilization of one component. In such a case the ratio ofintroduced non-activated to activated end groups can be favorablyincreased by melt mixing the activated and non-activated aromaticcarbonates together in the presence of a small amount of basic catalyst(such as tetramethylammonium hydroxide) to produce a scrambled productconsisting of a pre-equilibrated or statistical mixture of carbonates.The invention will now be further described with reference to thefollowing non-limiting examples.

Starting Material Polycarbonate

In all examples, either starting polycarbonate grade A, B, C, or D wasused. The starting materials were prepared by a melt process in acontinuous reactor system with the following properties:

Poly- Poly- Poly- Poly- carbonate A carbonate B carbonate C carbonate DWeight-average 18.3 × 10³ 8.11 × 10³ 30.5 × 10³ 4.6 × 10³ molecularg/mole g/mole g/mole* g/mole weight Mw: Number-aver- 8.34 × 10³ 4.05 ×10³ 14.1 × 10³ 2.5 × 10³ age molecular g/mole g/mole g/mole* g/moleweight Mn: Free OH 670 ppm 4020 ppm 834 ppm 7030 ppm content: End-capratio 84.6% 52.1% 81.0% 47.6% *Polystyrene standards

In the Examples, the following measurements were made. a) Molecularweight: Mw and Mn were measured by GPC analysis of 1 mg/ml polymersolutions in methylene chloride versus polystyrene standards. Unlessotherwise stated the measured polycarbonate Mw and Mn values were thencorrected for the difference in retention volume between polycarbonateand polystyrene standards.

b) Free-OH content was measured by UV/Visible analysis of the complexesformed from the polymer with TiCl₄ in methylene chloride solution. Insome cases the Free OH content was measured by direct infrared or ³¹ PNMR methods.

c) End-cap levels were calculated from the free OH content and Mnvalues.

d)Incorporation levels of specific end groups were determined by NMR.

e)The glass transition temperature of some end-capped polycarbonates wasmeasured by differential scanning calorimetry.

EXAMPLE 1

Symmetrical carbonates (both activated and non-activated) can beprepared using a method in which a substituted phenol with phosgene.This reaction is exemplified herein by the synthesis of methylsalicylate carbonate which was synthesized as follows.

A 500 ml, 5-neck Morton flask equipped with an overhead stirrer, a gasinlet tube connected to a phosgene cylinder through a flow controlvalve, a reflux condenser, a pH probe and a base addition tube ischarged with 0.2 mol of methyl salicylate, 120 ml of methylene chloride,90 ml of water and 0.1-1% of a condensation catalyst (for exampletriethylamine or a quaternary ammonium halide salt). Phosgene isintroduced at a rate of about 0.6 g/min and the pH of the aqueous phaseis maintained at about 10.0 by addition of 50% sodium hydroxide solutionthrough the base addition tube. When a total of 1.25 equivalentsofphosgene has been added, a nitrogen purge is introduced through thephosgene delivery tube and continued for 10 minutes until no phosgene orchloroformate is detected suing 4-nitrobenzylpyridine test paper ineither the overhead steam or the reaction solution. The aqueous phase isdiscarded and the organic phase is washed twice with 100 ml of 10% HClfollowed by washed with 100 ml of water until the washings are neutral.Solvent is removed on a rotary evaporator and the residue is purified bycrystallization from ethanol or a mixture of methylene chloride andhexane or heptane. The yield is typically 75-95% depending onrecrystallization efficiency.

EXAMPLE 2

A batch reactor tube was charged with 25 g of the pre-formedpolycarbonate A, 0.1160 g (ratio end-capper to free OH=0.55) diphenylcarbonate and 0.1790 g (ratio end-capper to free OH=0.55) bis-(methylsalicyl) carbonate under nitrogen. The mixture was heated to atemperature of 300° C. and stirred for 20 minutes. After the melt mixingstage, vacuum was applied to the system to a pressure of 0.5 mbar, andthe reaction continued for 20 minutes. After the reaction, the polymerwas sampled from the reaction tube. As summarized in Table 2, theend-cap ratio of the sample polycarbonate A was increased from 84.6% to93.1%.

EXAMPLE 3

A batch reactor tube was charged with 50 g of the pre-formedpolycarbonate A, 0.3481 g (ratio end-capper to free OH=0.825) diphenylcarbonate and 0.1790 g (ratio end-capper to free OH=0.275) bis-(methylsalicyl) carbonate under nitrogen. The mixture was heated to atemperature of 300° C. and stirred for 20 minutes. After the melt mixingstage, vacuum was applied to the system to a pressure of 0.5 mbar, andthe reaction continued for 20 minutes. After the reaction, the polymerwas sampled from the reaction tube. As summarized in Table 2, theend-cap ratio of the sample polycarbonate was increased from 84.6% to89.0%.

EXAMPLE 4

A batch reactor tube was charged with 50 g of the polycarbonate A,0.1160 g (ratio end-capper to free OH=0.275) diphenyl carbonate and0.1790 g (ratio end-capper to free OH=0.275) bis-(methyl salicyl)carbonate and 0.2950 g (ratio end-capper to free OH=0.55) methyl salicylphenyl carbonate under nitrogen. The mixture was heated to a temperatureof 300° C. and stirred for 20 minutes. After the melt mixing stage,vacuum was applied to the system to a pressure of 0.5 mbar, and thereaction continued for 20 minutes. After the reaction, the polymer wassampled from the reaction tube. As summarized in Table 2, the end-capratio of the sample polycarbonate was increased from 84.6% to 92.7%.

EXAMPLE 5

A batch reactor tube was charged with 25 g of the pre-formedpolycarbonate B, 0.4436 g(ratio endcapper to free-OH=0.5)di(meta-pentadecylphenyl) carbonate and 0.2307 g(ratio endcapper tofree-OH=0.5) bis-(methyl salicyl) carbonate under nitrogen. The mixturewas heated to a temperature of 300° C. and stirred for 20 minutes. Afterthe melt mixing stage vacuum was applied to the system to a pressure of0.5 mbar, and the reaction continued for 20 minutes. After the reactionthe polymer was sampled from the reaction tube. As summarized in Table2, the endcap ratio of the sample polycarbonate was increased from 52.1%to 87.2% and 1.12 mole percentage of the meta-pentadecylphenol wasincorporated in the end-capped polymer.

EXAMPLE 6

A batch reactor tube was charged with 25 g of the pre-formedpolycarbonate B, 0.4436 g(ratio endcapper to free-OH=0.5)di(meta-pentadecylphenyl) carbonate and 0.3370 g(ratio endcapper tofree-OH=0.5) bis-(benzyl salicyl) carbonate under nitrogen. The mixturewas heated to a temperature of 300° C. and stirred for 20 minutes. Afterthe melt mixing stage vacuum was applied to the system to a pressure of0.5 mbar, and the reaction continued for 20 minutes. After the reactionthe polymer was sampled from the reaction tube. As summarized in Table2, the endcap ratio of the sample polycarbonate was increased from 52.1%to 83.3% and 0.72 mole percentage of the meta-pentadecylphenol wasincorporated in the end-capped polymer.

EXAMPLE 7

A batch reactor tube was charged with 25 g of the pre-formedpolycarbonate B, 0.3139 g(ratio endcapper to free-OH=0.5)di(para-cumylphenyl) carbonate and 0.3370 g(ratio endcapper tofree-OH=0.5) bis-(benzyl salicyl) carbonate under nitrogen. The mixturewas heated to a temperature of 300° C. and stirred for 20 minutes. Afterthe melt mixing stage vacuum was applied to the system to a pressure of0.5 mbar, and the reaction continued for 20 minutes. After the reactionthe polymer was sampled from the reaction tube. As summarized in Table2, the endcap ratio of the sample polycarbonate was increased from 52.1%to 84.6% and 0.69 mole percentage of the p-cumylphenol was incorporatedin the end-capped polymer.

EXAMPLE 8

A batch reactor tube was charged with 25 g of the pre-formedpolycarbonate B, 0.3139 g(ratio endcapper to free-OH=0.5)di(para-cumylphenyl) carbonate and 0.2307 g(ratio endcapper tofree-OH=0.5) bis-(methyl salicyl) carbonate under nitrogen. The mixturewas heated to a temperature of 300° C. and stirred for 20 minutes. Afterthe melt mixing stage vacuum was applied to the system to a pressure of0.5 mbar, and the reaction continued for 20 minutes. After the reactionthe polymer was sampled from the reaction tube. As summarized in Table2, the endcap ratio of the sample polycarbonate was increased from 52.1%to 85.9% and 0.86 mole percentage of the p-cumylphenol was incorporatedin the end-capped polymer.

Comparative Example 1

Example 2 was repeated, but instead of diphenylcarbonate and bis-(methylsalicyl) carbonate no end-capper was charged to the reactor tube. Theresults are summarized in Table 2.

Comparative Example 2

Example 2 was repeated, but instead of diphenylcarbonate and bis-(methylsalicyl) carbonate, 0.2321 g (ratio endcapper/Free OH=1.1) diphenylcarbonate was charged to the reactor tube. The results are summarized inTable 2.

Comparative Example 3

Example 2 was repeated, but instead of diphenylcarbonate and bis-(methylsalicyl) carbonate, 0.2950 g (ratio endcapper/Free OH=1.1) methylsalicyl phenyl carbonate was charged to the reactor tube. The resultsare summarized in Table 2.

Comparative Example 4

Example 5 was repeated but instead of using di(meta-pentadecylphenyl)carbonate and bis-(methyl salicyl) carbonate no endcapper is charged tothe reactor tube. The results are summarized in Table 2.

Comparative Example 5

Example 5 was repeated but instead of using di(meta-pentadecylphenyl)carbonate and bis-(methyl salicyl) carbonate 0.4436 g (ratio endcapperto free-OH=0.5) of di(meta-pentadecylphenyl) carbonate is charged to thereactor tube. The results are summarized in Table 2.

Comparative Example 6

Example 5 was repeated but instead of using di(meta-pentadecylphenyl)carbonate and bis-(methyl salicyl) carbonate 0.2127 g (ratio endcapperto free-OH=0.5) of meta-pentadecylphenol is charged to the reactor tube.The results are summarized in Table 2.

EXAMPLE 9

End-capping reagent was prepared from a mixture of DPC and bis-(methylsalicyl) carbonate (MSC) such that the mole fraction of MSC was 0.33.The solvent was 1/1 acetone/toluene and the concentration of all specieswas 0.375 molar. A 1 ml aliquot contained a molar amount of carbonatesequivalent to the moles of free OH groups in 20 grams of the preformedpolycarbonate A.

A melt transesterification reaction was carried out in a 100 milliliterglass batch reactor equipped with a solid nickel helical agitator. Thereactor was charged with Polycarbonate A. No catalyst was added as therewas residual catalyst present in the resin ([Na_(active)]=50-150 ppb).The reactor was then assembled, sealed and the atmosphere was exchangedwith nitrogen. The reactor was brought to near atmospheric pressure andsubmerged into a fluidized bath which was at 300° C. After 15 minutes,agitation was begun at 150 rpm. After an additional five minutes, thereactants were fully melted and a homogeneous mixture was obtained. Atthis point, 2 ml of the endcapping solution was injected into the melt.The pressure was then reduced to 1 mm Hg. After 30 minutes, the reactorwas removed from the sand bath, and the melt was pulled from the reactorand dropped into liquid nitrogen to quench the reaction.

Mn (number average molecular weight) was obtained by GPC analysis of theend-capped polycarbonate. End-group concentration was determined by IRanalysis of the polymer end-groups. Results are presented in Table 2.

EXAMPLE 10

Example 9 was repeated but instead of using a mole fraction of 0.33 amole fraction of 0.66 MSC was used. The results are summarized in Table2.

Comparative Example 7 Example 9 was repeated but instead of using a molefraction of 0.33 no MSC was used. The results are summarized in Table 2.Comparative Example 8

Example 9 was repeated but instead of using a mole fraction of 0.33 amole fraction of 1.0 MSC was used. The results are summarized in Table2.

EXAMPLE 11

A melt transesterification reaction was carried out in a 90 milliliterstainless steel batch reactor equipped with a stainless steel twistedstir paddle and borosilicate glass reactor head. The polycarbonate C(31.0 g), bis(methyl salicyl) carbonate (0.49 g, molar ratio to freeOH=0.98), and di(meta-pentadecylphenyl) carbonate (0.46 g, molar ratioto free OH=0.48) were quickly added into the well of the preheatedreactor (180° C). Then the reactor was then placed a 300° C. pre-heatedaluminum jacket and the remaining apparatus assembled under gentle argonpurge. Initially, the melt was thermally equilibrated by stirring at 10to 80 RPM under a slight argon purge for 5 minutes. Then, while stirringat 40 to 80 RPM, the pressure over the reactor was reduced over 5minutes to 0.5 to 2 Torr. Stirring continued for 20 minutes under vacuumand volatiles were collected in a cold-trap. The reactor was thenrepressurized with argon and molten resin immediately ejected from thebottom of the reactor onto a collection tray.

A resin was obtained and characterized as follows: Mw=28851, Mn=13571(PS standards), Tg=134° C. Subsequent reprecipitation (using methanoland methylene chloride) provided a white powder with incorporatedalkylphenol endcapper=0.98 mol %, free OH level=278 ppm, and endcaplevel=93%.

EXAMPLE 12

Example 11 was repeated but the stainless steel batch reactor wascharged instead with polycarbonate C (30 g), bis(methyl salicyl)carbonate (0.97 g, molar ratio to free OH=1.99), and di(octadecylphenyl)carbonate (0.53 g, molar ratio to free OH=0.50). An end-capped resin wasobtained and characterized as follows: Mw=27796 and Mn=13791 (PSstandards), Tg=132° C. Subsequent reprecipitation provided a whitepowder with incorporated alkylphenol endcapper=0.58 mol %, free OHlevel=19 ppm, and endcap level=99.5%.

EXAMPLE 13

Example 11 was repeated but the stainless steel batch reactor wascharged instead with polycarbonate C (30 g), bis(methyl salicyl)carbonate (0.97 g, molar ratio to free OH=2.0), anddi(4-tert-butylphenyl) carbonate (0.24 g, molar ratio to free OH=0.50).An end-capped resin was obtained and characterized as follows: Mw=27361and Mn=13520 (PS standards), Tg=137° C. Subsequent reprecipitationprovided a white powder with incorporated alkylphenol endcapper=1.51 mol%, free OH level=16 ppm, and endcap level=99.6%.

EXAMPLE 14

Example 11 was repeated but the stainless steel batch reactor wascharged instead with polycarbonate C (30 g), bis(methyl salicyl)carbonate (0.97 g, molar ratio to free OH=2.0), anddi(meta-pentadecylphenyl) carbonate (0.47 g, molar ratio to freeOH=0.50). An end-capped resin was obtained and characterized as follows:Mw=30814 and Mn=15210 (PS standards), Tg=136° C. Subsequentreprecipitation provided a white powder with incorporated alkylphenolendcapper=1.08 mol %, free OH level=54 ppm, and endcap level=98.6%.

Comparative Example 9

Example 12 was repeated but the stainless steel batch reactor wascharged instead with polycarbonate C (31 g), bis(methyl salicyl)carbonate (0.49 g, molar ratio to free OH=0.98), and no bis-alkylphenylcarbonate endcapper. An end-capped resin was obtained and characterizedas follows: Mw=39090 and Mn=17314 (PS standards), Tg=145° C.,incorporated methyl salicylate endcapper=0.74 mol %, free OH level=37ppm, and endcap level=98.9%.

Comparative Example 10

Example 12 was repeated but the stainless steel batch reactor wascharged instead with polycarbonate C (31 g), no bis(methyl salicyl)carbonate, and di(meta-pentadecylphenyl) carbonate (0.46 g, molar ratioto free OH=0.48). An end-capped resin was obtained and characterized asfollows: Mw=31358 and Mn=14138 (PS standards), Tg=133° C., incorporatedalkylphenol=1.22 mol %, free OH level=450 ppm, and endcap level=89%.

EXAMPLE 15

In this example, a continuous reaction system was used. The apparatusconsists of one monomer mix agitation tank, two pre-polymerization tanksand one horizontally agitated polymerization tank. Bisphenol A anddiphenyl carbonate in a molar ratio of 1.08:1 were continuously suppliedto a heated agitation tank where a uniform solution was produced. About250 eq (2.5*10⁻⁴ mol/mol bisphenol A) tetramethylammonium hydroxide and1 eq (1.10⁻⁶ mol/mol bisphenol A) of NaOH were added to the solution ascatalysts in the first pre-polymerization tank. The solution was thensuccessively supplied to the next pre-polymerization tank and thehorizontally agitated polymerization tank, arranged in sequence, and thepolycondensation was allowed to proceed to produce a starting polymer“D” emerging from the outlet stream of the second pre-polymerizationtank for Example 14 with a Mw of 4439+289 g/mol, an Mn of 2407+121g/mol, and an endcap level of about 48%.

For example 15, a 50:50 molar ratio of diphenylcarbonate and bis-(methylsalicyl) carbonate was added by means of a heated static mixer to themolten polymer outlet stream of the pre-polymerization tanks (inletstream of the horizontally agitated polymerization tank) in an amount of1.97 mass % relative to the molten polymer stream.

EXAMPLE 16

A repeat of Example 15 except that a 50:50 molar ratio ofdiphenylcarbonate and bis-(benzyl salicyl) carbonate in an amount of2.55 mass % relative to the molten polymer stream was used.

TABLE 1 Examples of Symmetrical Activated Carbonates Structure Name(abbreviation) Data

Bis-methylsalicylate carbonate (bMSC) MW = 330 mp109° C.¹

BPA-bis-methylsalicylate carbonate MW = 572

Bis-ethyl salicylate carbonate MW = 358

Bis-propyl salicylate carbonate (bPrSC) MW = 386 mp = 57-58° C.

bis-2-benzoylphenyl carbonate MW = 442 mp = 111-112° C.

Bis-phenyl salicyl carbonate (bPhSC) MW = 454

Bis-benzyl salicyl carbonate (bBSC) MW = 482 mp = 68.5-71° C.

TABLE 2 Examples of End-Capping Endcapper Mw Mn Endcap PC EC/Free-OH (g/(g/ ratio mole % Example Type Name and structure mole-ratio mole) mole)(%) Incorp. Example 2 A Di-phenyl carbonate + 0.66 21006 9233 93.1 NAbis-methyl salicyl carbonate 0.55

Example 3 A Di-phenyl carbonate + 0.825 20121 8936 89.0 NA bis-methylsalicyl carbonate 0.275 Example 4 A Di-phenyl carbonate + 0.275 196908782 92.7 NA bis-methyl salicyl carbonate 0.275 Methyl salicyl phenylcarbonate 0.55 Example 5 B Di(m-pentadecylphenyl) Carbonate + 0.5 181239112 87.2 1.12 bis-benzyl salicyl carbonate 0.5 Example 6 BDi(m-pentadecylphenyl) Carbonate + 0.5 16144 8452 83.3 0.72 bis-benzylsalicyl carbonate 0.5

Example 7 B bis-benzyl salicyl carbonate + 0.5 19968 11081  84.6 0.69di-p-cumylphenylcarbonate 0.5

Example 8 B bis-benzyl salicyl carbonate + 0.5 20092 10563  85.9 0.86di-p-cumylphenylcarbonate 0.5 Comparative A — — 20992 11740  85.1 NAexample 1 Comparative A Di Phenyl Carbonate 1.1 21058 11692  88.1 NAexample 2

Comparative A Methyl Salicyl Phenyl Carbonate 1.1 18591 8233 92.0 NAexample 3

Comparative B — — 17870 8171 75.9 NA example 4 Comparative BDi(m-pentadecylphenyl) Carbonate 0.5  9712 5152 70.6 0.48 example 5

Comparative B m-pentadecylphenyl 0.5  9674 5257 68.4 0.32 example 6

Example 9 A Di-phenyl carbonate + 1.34 21193 9601 92.5 NA bis-methylsalicyl carbonate 0.66 Example 10 A Di-phenyl carbonate + 0.66 214629727 96.7 NA bis-methyl salicyl carbonate 1.34 Comparative A Di-phenylcarbonate + 2 19894 9043 87.7 NA example 7 bis-methyl salicyl carbonate0 Comparative A Di-phenyl carbonate + 0 22059 9975 99.4 NA example 8bis-methyl salicyl carbonate 2 Example 11 C Di(m-pentadecylphenyl)Carbonate + 0.48 21462 9727 96.7 0.98 bis-methyl salicyl carbonate 0.98Example 12 C Di(octatadecylphenyl) Carbonate + 0.5 27796 13791 99.5 0.58

bis-methyl salicyl carbonate Example 13 C Di(t-butylphenyl) Carbonate +0.5 27361* 13520* 99.6 1.51

bis-methyl salicyl carbonate Example 14 C Di(m-pentadecylphenyl)Carbonate + 0.5 30814* 15210* 98.6 1.06 bis-methyl salicyl carbonate 2.0Comparative C bis-methyl salicyl carbonate 0.96 39090* 17314* 98.9 0.74example 9 Comparative C Di(m-pentadecylpohenyl) Carbonate 0.48 31358*14138* 89.0 1.22 example 10 Example 15 D di-phenyl carbonate + — 15.57.19 82.7 NA bis-methyl salicyl carbonate — Example 16 D

— 16.0 7.17 82.8 NA Comparative D — — 16.2 7.32 45.8 NA Example 11*Molecular weights versus polystyrene standards.

What is claimed is:
 1. A method for preparing an end-cappedpolycarbonate resin, comprising the step of processing a mixturecomprising a polycarbonate having free hydroxyl-end groups and anend-capping reagent in a melt transesterification reaction to produce apolycarbonate resin, wherein the end-capping reagent comprises a mixtureof: (a) at least one species of a symmetrical activated aromaticcarbonate, and (b) at least one species of a symmetrical or asymmetricalnon-activated aromatic carbonate, whereby said end-capping reagentreacts with at least some of the free hydroxyl end-groups of thepolycarbonate to produce an end-capped polycarbonate resin.
 2. Themethod of claim 1, wherein the end-capping reagent contains theactivated and non-activated aromatic in a ratio of from 10:90 to 90:10.3. The method of claim 2, wherein the end-capping reagent is added in anamount such that the mole ratio of total carbonate in the end-cappingreagent to free-hydroxyl end groups is from 0.5 to 2.0.
 4. The method ofclaim 1, wherein the end-capping reagent is added in an amount such thatthe mole ratio of total carbonate in the end-capping reagent tofree-hydroxyl end groups is from 0.5 to 2.0.
 5. The method of claim 1,wherein the end-capping reagent comprises as a symmetrical activatedaromatic carbonate a compound of the formula:

wherein R is an electronegative substituent.
 6. The method of claim 5,wherein the electronegative substituents R are selected from among nitrogroups, halo groups, and carbonyl-containing groups.
 7. The method ofclaim 6, wherein the electronegative substituents R are selected fromamong methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, phenylcarbonyl,phenoxycarbonyl, and benzyloxycarbonyl.
 8. The method of claim 7,wherein the electronegative substituents R is methoxycarbonyl.
 9. Themethod of claim 6, wherein the end-capping reagent contains theactivated and non-activated aromatic carbonate in a ratio of from 10:90to 90:10.
 10. The method of claim 9, wherein the end-capping reagent isadded in an amount such that the mole ratio of total carbonate in theend-capping reagent to free-hydroxyl end groups is from 0.5 to 2.0. 11.The method of claim 6, wherein the end-capping reagent is added in anamount such that the mole ratio of total carbonate in the end-cappingreagent to free-hydroxyl end groups is from 0.5 to 2.0.
 12. The methodof claim 5, wherein the end-capping reagent comprises as a symmetricalnon-activated aromatic carbonate a compound of the formula:

wherein R₂ is selected from the group consisting of hydrogen and aC₁-C₃₆ alkyl, C₂-C₃₆ alkoxy group, C₆-C₃₆. aryl, C₇-C₃₆ aralkyl, andC₇-C₃₆ aralkyloxy and n is selected from the integers 1-5.
 13. Themethod of claim 12, wherein the end-capping reagent comprises as asymmetric non-activated aromatic carbonate a compound selected fromamong diphenylcarbonate, di-p-cumylphenylcarbonate,di-(tert-butylphenyl)carbonate, di-(octadecylphenyl)carbonate,di-(nonylphenyl) carbonate, di-(dodecyl phenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 14. The method of claim 13, wherein theend-capping reagent comprises as a symmetric non-activated aromaticcarbonate a compound selected from among diphenylcarbonate,di-p-cumylphenylcarbonate, di-(tert-butylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 15. The method of claim 1, wherein theend-capping reagent comprises as a symmetrical activated aromaticcarbonate a compound of the formula:

wherein R is an electronegative substituent.
 16. The method of claim 15,wherein the electronegative substituents R are selected from among nitrogroups, halo groups, and carbonyl-containing groups.
 17. The method ofclaim 16, wherein the end-capping reagent contains the activated andnon-activated aromatic carbonate in a ratio of from 10:90 to 90:10. 18.The method of claim 17, wherein the end-capping reagent is added in anamount such that the mole ratio of total carbonate in the end-cappingreagent to free-hydroxyl end groups is from 0.5 to 2.0.
 19. The methodof claim 18, wherein the end-capping reagent is added in an amount suchthat the mole ratio of total carbonate in the end-capping reagent tofree-hydroxyl end groups is from 0.5 to 2.0.
 20. The method of claim 15,wherein the end-capping reagent comprises as a symmetrical non-activatedaromatic carbonate a compound of the formula:

wherein R₂ is selected from the group consisting of hydrogen and aC₁-C₃₆ alkyl, C₁-C₃₆ alkoxy group, C₆-C₃₆. aryl, C₇-C₃₆ aralkyl, andC₇-C₃₆ aralkyloxy and n is selected from the integers 1-5.
 21. Themethod of claim 20, wherein the end-capping reagent comprises as asymmetric non-activated aromatic carbonate a compound selected fromamong diphenylcarbonate, di-p-cumylphenylcarbonate,di-(tert-butylphenyl)carbonate, di-(octadecylphenyl)carbonate,di-(nonylphenyl) carbonate, di-(dodecylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 22. The method of claim 21, wherein theend-capping reagent comprises as a symmetric non-activated aromaticcarbonate a compound selected from among diphenylcarbonate,di-p-cumylphenylcarbonate, di-(tert-butylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 23. The method of claim 1, wherein theend-capping reagent comprises as a symmetrical non-activated aromaticcarbonate a compound of the formula:

wherein R₂ is selected from the group consisting of hydrogen and aC₁-C₃₆ alkyl, C₁-C₃₆ alkoxy group, C₆-C₃₆. aryl, C₇-C₃₆ aralkyl, andC₇-C₃₆ aralkyloxy and n is selected from the integers 1-5.
 24. Themethod of claim 23, wherein the end-capping reagent comprises as asymmetric non-activated aromatic carbonate a compound selected fromamong diphenylcarbonate, di-p-cumylphenylcarbonate,di-(tert-butylphenyl)carbonate, di-(octadecylphenyl)carbonate,di-(nonylphenyl) carbonate, di-(dodecylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 25. The method of claim 24, wherein theend-capping reagent comprises as a symmetric non-activated aromaticcarbonate a compound selected from among diphenylcarbonate,di-p-cumylphenylcarbonate, di-(tert-butylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 26. The method according to claim 1,wherein the end-capping reagent is added to the polycarbonate in areactor system of the continuous or semi-continuous type.
 27. Theprocess according to claim 26, wherein the reactor system consists oftwo or more reactors in series.
 28. The process according to claim 27,wherein the end-capping reagent is added to the polycarbonate using astatic mixer.
 29. The process according to claim 1, wherein the formedpolycarbonate has a content of ortho-substituted phenols generated inthe terminal blocking reaction of 500 ppm or below.
 30. The processaccording to claim 1, wherein the formed polycarbonate has a content ofortho-substituted phenols generated in the terminal blocking reaction of100 ppm or below.
 31. The process according to claim 1, wherein theformed polycarbonate has a content of end-capping reagent of 500 ppm orbelow.
 32. The process according to claim 1, wherein the formedpolycarbonate has a content of end-capping reagent of 100 ppm or below.33. The process according to claim 1, wherein the formed polycarbonatehas a content of terminal 2-(alkoxycarbonyl)phenyl,2-(phenoxycarbonyl)phenyl, 2-(benzyloxycarbonyl)phenyl, and2-benzoylphenyl groups of 5,000 ppm or below.
 34. The process accordingto claim 1, wherein the formed polycarbonate has a content of terminal2-(methoxycarbonyl)phenyl groups of 2,500 ppm or below.
 35. The processaccording to claim 1, wherein the formed polycarbonate has a content ofterminal 2-(methoxycarbonyl)phenyl groups of 1,000 ppm or below.
 36. Anend-capping reagent consisting essentially of a mixture of: (a) one ormore species of symmetrical activated aromatic carbonate, and (b) one ormore species of a symmetrical non-activated aromatic carbonate,optionally in solvent.
 37. The reagent of claim 36, wherein theend-capping reagent includes as a symmetrical activated aromaticcarbonate a compound of the formula:

wherein R is an electronegative substituent.
 38. The reagent of claim37, wherein the electronegative substituents R are selected from amongnitro groups, halo groups, and carbonyl-containing groups.
 39. Thereagent of claim 38, wherein the electronegative substituents R areselected from among methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl,phenylcarbonyl, phenoxycarbonyl, and benzyloxycarbonyl.
 40. The reagentof claim 39, wherein the electronegative substituents R ismethoxycarbonyl.
 41. The reagent of claim 38, wherein the end-cappingreagent contains the activated and non-activated aromatic carbonates ina ratio of from 10:90 to 90:10.
 42. The reagent of claim 38, wherein theend-capping reagent comprises as a symmetrical non-activated aromaticcarbonate a compound of the formula:

wherein R₂ is selected from the group consisting of hydrogen and aC₁-C₃₆ alkyl, C₁-C₃₆ alkoxy group, C₆-C₃₆. aryl, C₇-C₃₆ aralkyl, andC₇-C₃₆ aralkyloxy and n is selected from the integers 1-5.
 43. Themethod of claim 42, wherein the end-capping reagent comprises as asymmetric non-activated aromatic carbonate a compound selected fromamong diphenylcarbonate, di-p-cumylphenylcarbonate,di-(tert-butylphenyl)carbonate, di-(octadecylphenyl)carbonate,di-(nonylphenyl) carbonate, di-(dodecylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 44. The method of claim 43, wherein theend-capping reagent comprises as a symmetric non-activated aromaticcarbonate a compound selected from among diphenylcarbonate,di-p-cumylphenylcarbonate, di-(tert-butylphenyl)carbonate,di-(3-pentadecylphenyl)carbonate (also called cardanol carbonate) anddi-(octadecylphenyl)carbonate.
 45. The reagent of claim 44, wherein theend-capping reagent contains the activated and non-activated aromaticcarbonates in a ratio of from 10:90 to 90:10.
 46. The reagent of claim36, wherein the end-capping reagent contains the activated andnon-activated aromatic carbonates in a ratio of from 10:90 to 90:10.